For 5G millimeter-wave (mmWave) mobile handsets, a reduced number of antenna arrays is desirable due to space limitations. The number of antenna arrays could equal the minimal number needed to satisfy equivalent isotropically radiated power (EIRP) requirements and obtain adequate angular coverage.
Unfortunately, display screens and batteries in a handset impose serious difficulties for mmWave antenna array allocation and signal routing. Traditional handset antenna designs use meandered electrical small antennas (ESA) that are conformal to part of the handset's case, which in low frequencies are adequate to obtain omni-directional coverage due to strong multipath/scattering effects and much lower gain requirements.
In mmWave antenna designs, big scatters such as display screens and batteries represent ultra-large ground planes for any radiators. Added material losses due to the reduced wavelengths of mmWave antennas make antenna efficiency an important factor in the design of mmWave antennas. The reduced wavelengths also make RF signal transitions from a microstrip to other transmission lines prone to radiation and reflection. These factors can result in a very limited selection of antenna elements for use in mmWave antennas. For these reasons, many mmWave antennas are designed directly on a printed circuit board (PCB), in-package, or on an integrated circuit (IC).
Existing PCB-compatible mmWave antenna designs typically use printed-dipole/loop, Yagi-Uda, slot, patch, or Vivaldi antenna elements. Of these five candidates, three are inherently directional with narrow beamwidths and relatively high gains. Dipole and slot antenna elements could be omni-directional in free space, but printed mmWave dipole and slot antenna elements become directional in a complex environment such as a handset chassis due to strong substrate and ground plane effects. Printed ultra-wideband (UWB) antennas are typically excluded due to their dimensions and unnecessarily wide bandwidths. Planar inverted F-antennas (PIFAs) and other popular printable ESA antennas are suitable for 3G/4G devices but typically lack the efficiency needed for 5G devices.